Though her ancestors left Scotland in the 12th century, Virginia resident Patricia Reynolds feels such strong kinship for the place that she has become a vocal supporter of Scottish independence.

"This is a nation going for self-determination, with the ability to use its natural resources to benefit its own people," said Ms. Reynolds, a University of Mary Washington assistant education professor. "If that isn't the American way, I don't know what is."

With Scotland set to vote Thursday on whether to end a 307-year-old union with the U.K., Scottish-Americans and Scottish expatriates across the U.S. also are watching—and debating—a campaign that polls suggest is too close to call.

In Scotland, the two sides in the referendum appealed for support Sunday. British Prime Minister
David Cameron,
who will speak in Scotland on Monday, is expected to make another plea to Scots to reject independence. "There's no going back from this. No re-run. If Scotland votes yes, the U.K. will split, and we will go our separate ways forever," he is expected to say, according to a transcript of excerpts from his speech.

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If Scotland votes for independence, just what will it be splitting from? Find out in this short animation.

A survey released late Saturday by ICM Research for the Telegraph newspaper put support for independence at 49% of those surveyed with 42% against. Two other surveys put support for the union at 47% in both, versus 43% or 41% and a survey by Panelbase for the Sunday Times also put the union camp slightly ahead.

Ms. Reynolds said she took up the cause after hearing Scottish National Party leader
Alex Salmond
make his case in a speech five years ago. She has a pro-independence sticker on her car and helps administer a Facebook page devoted to the issue.

In Houston, Heather Smith Vacker, a Scottish-American teacher, said a yearning to separate from England has been passed down through the generations since her ancestors emigrated to the U.S. But Ms. Vacker, 50, said she isn't sure Scotland is prepared to be its own country.

About 5.4 million Americans report Scottish ancestry, 2012 U.S. Census estimates show, and 3.1 million identify as Scotch-Irish-Scots who came via Ireland. Census officials said people can check both categories.

While many Scottish-Americans may pay scant attention to the politics of an ancient homeland, one who closely follows events there is Rep.
John Duncan Jr.
, a Tennessee Republican who co-chairs the Friends of Scotland Caucus in the House.

"I'm very much pro-independence," he said. "If they follow free-market, small-government policies, they could become very, very prosperous and be a wonderful place."

The debate is roiling Scottish expatriates in the U.S. The referendum is too touchy to discuss with friends, said
Alan Bain,
the 78-year-old chairman of the New York-based American-Scottish Foundation, which is neutral. For most expats, he said, the passion is fueled by an emotional attachment and historical precedence.

But he has heard concerns ranging from a possible impact on pension values to the fate of the Union Jack, the U.K.'s national flag. "This is not a 'Braveheart' moment," said Mr. Bain, who moved to New York in 1961. "This is something that is very significant for a whole swath of people, not just Scots."

Matt Girvan, who grew up in Scotland and moved to the U.S. eight years ago, said he has spent the past year trying to convince his family to vote yes on his behalf. Nonresidents can't cast ballots.

"Our history of independence is too strong; we even managed to stay independent of the Roman Empire," said Mr. Girvan, a 35-year-old technology entrepreneur living in Truckee, Calif.

Paul McGinlay, vice president of a seller of Scottish whiskey and clothing who lives in Belmont, Calif., said with a laugh that if the independence side wins, "I'll put on a kilt and go partying with a few people."

Other expats say their homeland should remain with the U.K.

"A small country like Scotland is probably going to do better with uniting," said Eddie Morgan, an Atlanta resident who was born and raised in Scotland and holds dual British and American citizenship. The Emory University pharmacology professor, 59, is past president of the Burns Club of Atlanta, dedicated to promoting Scottish culture and the literature of Scottish poet Robert Burns.

The vote has divided his family, with he and his mother, who still lives in Scotland, supporting "no," and two siblings backing "yes." Mr. Morgan described himself as nationalistic but worries that a split could disturb Scotland's economy.

Alan Purves, who moved to the U.S. in 1979, said there are too many unanswered questions concerning sovereignty, such as what form of currency would be used. Independence backers want to keep the pound sterling and form a currency union with the U.K. Without one, an independent Scotland could use the pound unilaterally, create a new currency or join the euro. Also, he said, supporters cite Scotland's big oil reserves as a source of future revenues, but those are declining.

In Chicago, interest is so keen that the Duke of Perth, a pub with an extensive Scotch selection and a Robert the Bruce burger, plans to break its no-television rule so patrons can watch the election returns.

Co-owner Jack Crombie, who moved to the U.S. from Scotland 30 years ago, said that while he is in the "aye" camp, the pub is staying out of it.

In Canada, which has a large and influential Scots diaspora, more than 15% of the population claims Scottish heritage. Every province has its own official tartan.

In Pictou, Nova Scotia, the self-professed "Birthplace of New Scotland," Ann Emmett says talk of the referendum has been mainly limited to the British tourists who visit her hotel and bar, the Braeside Country Inn.

Kilt-maker Margaret Struth-Gaff says few Canadians have been talking about the referendum in her Ontario shop, Burnett's & Struth Scottish Regalia Ltd, which sells all-things tartan to Canadians.

Drinkers at the Loose Cannon Scottish Public House in Canada's eastern province of Nova Scotia were alive with discussion: on baseball and hockey.

Officially, Canada is in the "No" camp. Prime Minister
Stephen Harper
and John Baird, his Minister of Foreign Affairs, have both said they would like to see the U.K. stay together. But the lack of fervor among Scottish-Canadians may have something to do with the fact that the families of most have been in Canada since the 19th century, said Professor Robert Young, who follows secession debates around the world from Western University in London, Ontario.

"We haven't had many Scots come lately…so I don't think you will see as active an involvement in Scots life" as you may with ethnic groups who have arrived more recently, he said.